Replacement therapy has been shown to work successfully in rodents for the prevention of dental caries. This application describes studies designed to develop replacement therapy for the prevention of dental caries in humans. The purpose of Aims 1 and 2 is to further understand the physiology and ecological parameters of Streptococcus mutans in order to construct an effector strain that is both non-virulent and capable of efficiently colonizing the human oral cavity. These studies will involve accurate analysis of the fermentation patterns of wild-type and lactate dehydrogenase-deficient S. mutans, and identification of heritable changes that occur during prolonged colonization of the human oral cavity which help to adapt S. mutans to that environment. From the information obtained, idealized effector strains will be isolated. They will be tested in Aim 3, using gnotobiotic and conventional rat models, for their cariogenic potential. The ability of the effector strains to preemptively colonize the oral cavities of rodents, and displace indigenous strains of S. mutans will be tested in Aim 4. A strain of S. mutans that produces an unusually potent bacteriocin will serve as the starting strain for these studies. In Aim 5 the minimum infection regimen required to obtain high levels of colonization by this strain in humans will be examined in a clinical pilot study involving 3 subjects. Microbiological analysis of saliva and plaque from these subjects will be used to determine the effects of the effector strain on indigenous S. mutans populations and on the general plaque microflora. The bacteriocin produced by this strain of S. mutans is of interest as a topical anti-caries agent. In Aim 6, methods for its purification, sequencing, and in vitro synthesis are developed, using both conventional biochemical and recombinant DNA technology. Aim 7 involves studies to directly demonstrate the role of the bacteriocin an an ecological determinant. The bacteriocin will also be tested in a rodent model for its ability to serve as a topical anti-caries agent.